(1) Field of the Invention
The present invention relates to the field of nuclear medicine systems. Specifically, the present invention relates to signal processing of signals from a scintillation detector within a gamma camera system.
(2) Prior Art
Gamma cameras (e.g., of the Anger type) include a scintillation detector composed of well known components including an array of photomultiplier tubes (PMTs), a crystal layer and a collimator. Gamma rays (events) that are radiated onto the crystal layer, through the collimator, create scintillations within the crystal layer that are detected by the PMT array. Each PMT of the array generates a separate signal or response that is called a channel signal. The channel signals from all of the PMTs of the array are coupled to processing logic that computes, among other values, the spatial coordinate of the gamma event as well as the energy associated with the gamma interaction. This data is then used for image generation by the gamma camera system.
The characteristic light collection of the PMT array of a gamma camera is dependent on the solid angled viewed by the PMT photocathode in terms of the origin of the point of light associated with a scintillation. The spatial computation (e.g., centroiding) of a gamma interaction is done by linear weighting of the output responses of each PMT channel in the PMT cluster according to well known mechanisms and methods. The spatial computation will be non-linear in nature because the central PMT is weighted too much due, in part, to the physical characteristics of the PMTs. To counterbalance for this effect, the PMT channel signals are compressed by a factor that is dependent on their amplitude. This compressing technique is often referred to as "Dynamic Compression," because a predetermined compression factor is applied to each PMT that contributes to a centroid calculation (e.g., of the PMT cluster).
In the prior art, this dynamic compression was performed utilizing specialized analog circuitry including break point driver circuits composed of diode networks. These break point driver circuits adjust the PMT channel signal differently over different signal ranges dependent, in part, on the total energy of the interaction and the signal value being corrected. However, this solution is not advantageous because the procedure utilized to perform the dynamic compression is implemented in analog hardware and is not readily modifiable within the camera system. Further, the break point driver circuits of the prior art do not offer a smooth transition from one signal range to another due to their hardware implementation, but rather correct the PMT's response abruptly and nonuniformly near the signal range transitions. The analog break point driver circuits of the prior art operate on analog and not digital data signals, therefore these do not operate on channel signals that have been digitized. Also, another disadvantage of the prior art break point circuits is that they are very temperature sensitive and their response is therefore not constant over operational temperature ranges of a gamma camera system.
Therefore, it would be advantageous to provide a dynamic compression table that is readily modifiable, does not have signal range transition nonuniformities, and further that can process digital data. The present invention offers such advantageous functionality.
Accordingly, it is an object of the present invention to improve image generation within a gamma camera utilizing improved signal processing circuitry and methods for use. It is another object of the present invention to provide a dynamic compression circuit implemented using a memory device such that the dynamic compression procedure is readily modifiable. It is yet another object of the present invention to provide a digital circuit for performing dynamic compression wherein the input to the circuit (e.g., the channel signal) is in digital form. It is yet another object of the present invention to provide a circuit for performing digital dynamic compression wherein the transitions from one signal range to another are performed smoothly and utilize a relatively continuous transformation procedure. These and other objects of the present invention not specifically mentioned above will become clear within discussions of the present invention to follow.